In Fourier modal methods like the RCWA and the Differential Method the Li-rules for products in truncated Fourier
space have to be obeyed in order to achieve good convergence of the results with respect to the mode number. The Lirules
have to be applied differently for parts of the field that are tangential and orthogonal to material boundaries. This is
achieved in the Differential Method by including a field of vectors in the calculation that are normal to the material
boundaries. The same can be done laterally in each layer of an RCWA calculation of a 2-D periodic structure. It turns out
that discontinuities in the normal vector field can disturb the computation especially when metallic materials are
dominant in the structure which would make the usefulness of the normal vector method questionable. So it is of great
importance to investigate how normal vector fields can be established with as few discontinuities as possible. We present
various methods for the 2-D RCWA and the 1-D and 2-D Differential Method and compare the respective convergence
behaviors. Especially we emphasize methods that are automatic and require as few user input as possible.